Abstract
Charge transport near the Dirac point (DP) was investigated in graphene using ferroelectric (FE) gating in the temperature range of 300 < T < 350 K. We observed that the conductivity (σ) near the DP had a positive temperature gradient that switched to a negative temperature gradient with increasing temperature. The switch to a negative temperature gradient shifted to higher temperatures and gradually weakened upon moving away from the DP. Impurity charge compensation via polarization of the FE together with a temperature-dependent graphene–impurity charge separation was proposed as being responsible for the non-monotonicity in σ(T). A self-consistent theory for graphene transport with impurity charge scattering and phonon scattering was used to analyze the results. Non-monotonic charge transport was also observed in the temperature dependence of the residual conductivity (σr). Theoretical analysis of both σ and σr revealed a temperature independent contribution of ∼1.16e2h that is probably inherent to pristine graphene.
Highlights
Graphene is a two-dimensional zero bandgap semi-metal consisting of planar sp2-bonded carbon atoms arranged in a hexagonal lattice.1 It possesses a linear energy dispersion relation and exhibits an ambipolar electric field effect.2–4 Pristine graphene is unstable in air as charged impurities are adsorbed during its growth and post processing.3,5–8 In substrate supported graphene, these impurity charges are randomly attached to its top surface and at the graphene/substrate interface
Our results show that the temperature dependence of σ at the Dirac point (DP) switched from having a positive temperature gradient to having a negative temperature gradient with increasing temperature
At and near the Dirac point, P approaches zero and doping by E was weak, given the low values of VG and the relatively thick polymer film. Four features in this figure are evident as temperature was increased: (i) DP-1 shifts toward VG = 0 V, i.e., p-doping weakens; (ii) the conductivity at the Dirac point shows a peak where there is a change in the sign of the slope of σ(T); (iii) the mobility increases; (iv) the width (ΔVmin) of the conductivity minimum at the DP-1 gets narrower; and (v) σr remains close to that at the DP and shows a similar temperature dependence due to the narrowness of the plateaus at σ vs VG curves
Summary
Graphene is a two-dimensional zero bandgap semi-metal consisting of planar sp2-bonded carbon atoms arranged in a hexagonal lattice. It possesses a linear energy dispersion relation and exhibits an ambipolar electric field effect. Pristine graphene is unstable in air as charged impurities are adsorbed during its growth and post processing. In substrate supported graphene, these impurity charges are randomly attached to its top surface and at the graphene/substrate interface. Temperature-dependent charge transport measurements of graphene on Si+/SiO2 substrates with back gating show that the conductivity (σ) exhibits a positive and a negative temperature gradient depending on the graphene quality and carrier charge concentration.. The activation mechanism is important at low impurity concentrations where the range of the potential fluctuations is higher, leading to a higher value of the activation energy.17 For this mechanism to strongly contribute to the charge transport, the activation energy must exceed the thermal energy as, for example, in the experiments reported previously.. The energy associated with carrier’s activation across the potential fluctuations appears to be of the order of a few milli-electron volts, which is much smaller than the thermal energy within the considered temperature range This gives us grounds to suggest that the graphene sample used in the experiments was a “dirty” one, characterized by a high ni, where the effect of potential fluctuations was rather small. We focus on diffusive transport that is mostly controlled by impurity charge scattering and phonon scattering
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